1. Field of the Invention
The present invention relates to electrical phase shifters and more specifically to electrically variable phase shifters for use in waveguide transmission systems.
2. Description of the Prior Art
A need frequently arises for variable phase shifters for use in microwave transmission systems.
For instance, there is presently a great deal of interest in developing electronically scanning antenna arrays to replace mechanically scanned antennas in existing microwave radar installations since it is believed that the performance, cost, and weight of the existing antennas could be improved significantly by using electronic scanning. It has been proposed that electronic scanning could be accomplished by utilizing a low loss, electrically variable phase shifter in the antenna's transmission system to accomplish this purpose.
Typically, antennas of the type under consideration may operate at 94 GH.sub.Z (W-band). Feritte phase shifter are known in the art which may be used at this frequency, but they exhibit high losses. Furthermore, the transverse dimensions of these devices are relatively large, typically being of the order of 2 cm in diameter. Since the distance between adjacent elements in W-band components may be in the order of 1.5 mm, the size of the ferrite devices makes it difficult to incorporate these devices into such arrays.
PIN diodes capable of operating at W-band frequencies are known in the art and may be employed with sections of waveguide to provide digital phase shifters. However, the insertion loss of these diodes is high and the diodes are susceptible to burnout at high power. For example, the insertion loss of a 4 bit PIN diode-waveguide phase shifter at millimeter wave frequencies, may be as high as 4-6 dB.
Still other approaches toward achieving the desired phase shift have been reported in the literature. Modulating the width of a waveguide with semiconductor, for instance, has been described in an article entitled "Millimeter Wave Phase Shifter" by B. J. Levin and G. G. Weidner and appearing on pp. 489-505 of the RCA Review, Vol. 34, 1973.
An article entitle "Optical Control of Millimeter Wave Propagation in Dielectric Waveguides" written by C. H. Lee et al and published in the IEEE Journal of Quantum Electronics, Vol. QE-16, NO. 3 for March 1980 describes a technique for modulating the conductivity of a semiconductor through the use of a laser.
Unfortunately, however, the results achieved using these latter two methods appear to indicate that these techniques also suffer from relatively high losses.
As opposed to the prior art devices, the insertion loss of a phase shifter constructed in accordance with the present invention typically displays an insertion loss of less than 0.5 dB for 360.degree. of a phase shift.
Furthermore, the transverse dimensions of the present phase shifter can be made as small as the width of a W-band waveguide (0.45 cm) so that the phase shifter can be readily incorporated in a planar array for electronic scanning.
In addition, the phase shifter of the present invention is relatively inexpensive, easy to fabricate, and requires very little driver power.